Marvel
DC
Naruto
One Piece
Dragon Ball Z,
Warhammer 40k
Star Wars
Invincible
These universes have merged into a single realm, each representing a distinct country and for whatever reason, they decide to go to war against each other. They all have basic knowledge of each other's skills, powers, resources, etc., and the heroes and villains from their respective country will all be working together.
(No supreme beings such as TOAA, The Presence, etc., are allowed to help. No reality warping, and no time travel is allowed.)
(Alliances between universes can be formed but no more than 1)
Edit: Characters with reality-altering abilities are permitted, but they are not allowed to use that specific power to directly influence events. A reality warper cannot simply alter reality to ensure victory. When I say "no supreme beings," I'm referring to characters who have unparalleled omnipotent power, like TOAA and The Presence.